A. Field of the Invention
This invention relates generally to the field of electrical signal coupling, and more particularly to circuits and methods of coupling optically sensitive scan detectors to amplification stages.
B. Description of the Related Art
Bar code is a prominent automatic identification technology used to collect information about persons, places, or things. Bar codes work much like Morse Code in which dots and dashes represent letters and numbers. However, unlike Morse Code, information in bar codes is symbolized by bars and the spaces separating the bars. The bars and spaces vary by thickness depending on the type of symbology. The ratio of the dimensions between wide elements and narrow elements in a bar code is typically referred to as the wide to narrow ratio. The dimension of the narrow bars and spaces is typically referred to as the “X” dimension, and the dimension of the wide bars and spaces is a multiple of the X dimension. The wide to narrow ratio is preselected according to the symbology.
Most bar code readers are comprised of a laser, an optical timing detector for synchronizing the start of each scan by the reader, a rotating multifaceted mirror for passing a beam produced by the laser across the bar code during a scan period and diverting light reflected back at the reader during the scan period towards an optically sensitive scan detector, the optically sensitive scan detector converting the reflected light into an equivalent electrical signal, a logic detector for converting the equivalent electrical signal into unique logic states, and a coupling for carrying the equivalent electrical signal from the scan detector to the logic detector. Typically, the optically sensitive scan detector includes a field effect transistor (FET) input gain stage that impresses a direct current (DC) offset on the detected signal. As a result, the most common coupling employed between the scan detector and the logic detector is a resistor-capacitor (RC) coupling because it can be designed to remove the DC offset created by the scan detector. FIG. 1 shows a common RC coupling configuration comprising a resistor and a capacitor operatively connected to each other. FIG. 2 shows an exemplary output signal of the RC coupling depicted in FIG. 1. Note the DC offset of the output signal relative, to 0 volts DC.
Two common conditions in the vicinity of the bar code can have a negative effect on the output signal of the scan detector. The first condition is a fluctuation in background light level. As the level of background light in the vicinity of the bar code changes the output signal level of the optical detector during the non-scan or no laser reflectance periods varies as well. Usually the effect of this first condition on the operation of the reader is negligible and may be ignored. The second condition is the degree of spectral reflectance from the background on which the bar code is printed or attached. The more reflective the background and/or the closer to perpendicular the angle of incidence of the laser beam with the target the higher the degree of spectral reflectance received by the optical detector. However, the amount of light reflected from the barcode itself during both the scan and non-scan periods is relatively independent of background type and remains essentially the same. This results in the average DC level of the optical detector output signal varying with the degree of spectral reflectance from the background. Moreover, an RC coupling between the optical detector and the logic detector will not remove the effects of the changes in the average DC level of the optical detector output signal. If the spectral reflectance is high enough the effect can be to shift that portion of the optical detector output signal containing the bar code information outside the logic detector window of operation, thereby impairing the operation of the bar code reader. This condition is sometimes referred to as retro-reflectance.
To prevent retro-reflectance and compensate for the effects of spectral reflectance in general, highly reflective backgrounds are avoided where possible and fixed-mount bar code readers are typically designed with the lasers being pitched or offset a few degrees from perpendicular. The number of degrees from perpendicular that a laser must be placed to prevent bar code reader impairment is sometimes referred to as the reader's specular reflection zone. There are instances however where very useful information may be obtained from light reflected by a target in the vicinity of a highly reflective background during a scan period. For instance, where bar codes are being used to track and identify biological samples such as blood as they travel through an automated analysis system, it has been determined by the assignee of the present invention that information about the sample vessel itself (size, shape, fluid level, cover, insert, and the like) may be obtained from laser light reflected by the sample vessel in the vicinity of a highly reflective background. Accordingly, it would be very advantageous to use the optical detector output signal of a single bar code scanner for both purposes. Moreover, it would be highly advantageous if bar code readers could be designed so that no pitch or offset from perpendicular of the laser were necessary.